Thermoplastics contain mineral fillers, eg. wollastonite, talc, chalk or quartz sand, in order to increase the rigidity and strength and to reduce the thermal elongation, shrinkage and warpage.
So long as the content of mineral fillers does not exceed a proportion of about 30 % by volume, incorporation into the molten plastics is not generally associated with any particular problems. At higher degrees of filling, however, the flow behavior of the materials becomes so poor that they become virtually impossible to process, for example by injection molding.
Recently, a process for the production of ceramic moldings in which a thermoplastic is mixed with ceramic powders, heated in a suitable machine, shaped and cooled, has become increasingly important. The plastic is subsequently removed, for example by heating, before the ceramic materials are finally sintered. In this process, thermoplastics which contain 50 % by volume or more of ceramic material are required. Only at this and higher degrees of filling is it possible to obtain, from the moldings after removal of the thermoplastic binder, reasonably solid preforms which survive the further process steps, in particular the sintering, in an undamaged condition.
The higher the degree of filling, the lower the shrinkage on sintering and the better the moldings retain their dimensions. For this reason, ceramic parts of the highest possible degree of filling by volume of the ceramic powder are desired for injection molding or extrusion. This also applies in essence to the production of ceramic fibers, one method for which is to mix ceramic powders with a polymer, to remove this mixture by dissolution, pyrolysis or hydrolysis and to sinter the "green" fiber. The highest possible degree of filling by volume and good spinning properties must be ensured here.
However, the preparation of such materials containing .gtoreq.50 % by volume of ceramic powders is associated with considerable problems if it is desired to mix these ceramic powders with the melt of a thermoplastic. A long mixing time is necessary to homogenize the material, while the powder is deagglomerated and individual powder particles are wetted by the polymer. It has become apparent that the deagglomeration of the powder is incomplete and it is hardly possible to achieve degrees of filling above 50 % by volume. Although the dispersion time is reduced somewhat by treating the powders with dispersion aids, such as polyethylene oxides, montan ester waxes, long-chain carboxylic acids or long-chain hydroxycarboxylic acids, the deagglomeration is not, however, promoted to any significant extent. Due to the long mixing times under steep shear gradients, the mixer materials, principally iron, are subjected to considerable erosion and the batch is thus contaminated.